Since the early days of photography, camera manufacturers have focused on fabricating the visual representation of human visual perspective to make photographs (“photos”) look as true to life as possible. Whether using a DSLR or smartphone camera, cameras are generally rectilinear. That is to say, cameras generally attempt to capture straight lines in a scene as straight lines in a photo, which helps the human eye perceive depth within a 2-D scene and creates an image that appears natural. This approach works well when a camera is pointed directly at (i.e., perpendicular to) the subject. However, as soon as the camera is tilted in any direction, the perspective or horizon in the photo is altered and the subject may appear tilted or otherwise skewed in the resulting photo. Similarly, holding a shaky camera can result in a tilted camera, causing the perspective or horizon in a photo to appear tilted or otherwise skewed.
Perspective distortion is particularly evident in photographs that contain straight lines or geometric shapes. For example, if a photographer attempts to capture a tall building in a photo, the photographer generally must tilt the camera upwards from his position on the ground to frame the entire building in the viewfinder. Although the entire building may fit within the frame, the sides of the building will appear slanted in the resulting photo (vertical distortion). This effect occurs because the top of the building is farther away from the camera lens than the bottom of the building. Similarly, if the camera is tilted to the right or left of the perpendicular line pointing the camera squarely at the face of the building, the top and bottom of the building will appear slanted in the resulting photo (horizontal distortion). Relatedly, if the photographer tilts the camera so it is not level with the scene (a change in camera roll, using the example coordinate system of FIG. 4), the photo can appear tilted. If the photo includes a horizon, the horizon in the photo can appear tilted. Generally, perspective distortion and horizon tilt are unpleasant because they make the scene in the photo look different than the way we experience the scene in the real world.